This season the Criminal Minds team has been stalked by someone called the Replicator, who takes old crimes they've solved and recreates them.
I assume the finale this season will reveal the Replicator's identity but I'm going to take a stab at guessing.
I think the Replicator is Gideon.
Gideon was a member of the team for the first two seasons, played by Mandy Patinkin. He was a brilliant profiler, able to creep into anyone's mind and outguess them. His girlfriend was violently murdered by a psychopath who made their games personal. (This seems to be a common fate for significant others of team members ... which is bad news for my crushes on Matthew Gray Gubler and Shemar Moore.) Gideon left the team (Patinkin went back to Broadway), saying he didn't want to look into the darkness anymore.
Here is my rationale:
The Replicator knows the team. There are other behavioural analysis units but the Replicator focused on this one. That makes it personal.
The Replicator is highly skilled at both manipulating others and staying unnoticed. He not only kills people himself but convinces other people to become serial killers to throw the team off his scent. That speaks to someone with amazing profiling skills.
The Replicator used the term "Zugzwang" which is a chess term. It's the point in the game at which check mate is inevitable. Gideon was a champion chess player, capable of beating resident genius, Dr. Reed.
And my final reason for believing it: it would be an amazingly cool twist in the series.
Have to wait until the end of the season to find out if I'm right.